In the manufacture of paper, the web emerging from the dryer section of the paper machine generally has an irregular moisture content in a direction transverse to the direction of web travel. For subsequent processing and testing operations, it is desirable that the web have as uniform a moisture profile in the cross web direction as is possible.
Various sytems have been devised in the prior art in attempts to maintain a relatively uniform cross web moisture content profile. In all of these systems, a cross web moisture content profile is taken and control signals derived therefrom are fed to a rewet actuator which applies moisture to the web.
In one fiber rewet actuator known to the art, solenoid type valves are used to control the flow of water through a spray nozzle onto the web. In an effort to achieve some measure of resolution, a number of solenoid valves configured in parallel are associated with each nozzle. Generally, this type of rewet actuator requires that four solenoid valves be provided per nozzle. This is independent of the number of data zones across the web. A single zone may have one, two or even three nozzles so that the number of solenoid valves required for the actuator is relatively large. Thus, actuators of this type are large and bulky and require an inordinate amount of machine floor space for cabinets containing the control valves. In addition, the large number of components required for these systems results in high failure rates and long troubleshooting periods. A typical system having one hundred data zones would have a minimum of four hundred solenoid valves.
In addition, in a system of the type described hereinabove, any customizing which must be done on-site results in a major down time of the system. Fine tuning of the system is lengthy and complex. Nozzles must be evenly matched in performance or any mismatch causes a notable error in the moisture profile.
In any system during a web break the spray boom does not spray any water. Where air atomizing nozzles are used the air continues to flow while the water is shut off. During the period when the water is shut off, the water supply units build up some pressure. When the web is back on line, the solenoid having the lowest flow rate in the parallel configuration is opened. Owing to the high pressure which has built up in the supply unit during the down time, the spray boom will always overshoot the amount of moisture required. Such overshoot continues until pressure in the water supply unit drops to the proper level.
As an alternate to the use of solenoid valves, it has been proposed to use a needle type valve with a stepper motor for positioning the valve shaft. Along with other problems, this arrangement suffers from low resolution and backlash.